This application claims priority of European Patent Application No. 99302696.2, which was filed on Apr. 07, 1999.
This invention relates to methods of filtering alarm indication signals (AIS) in communications systems such as Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) systems.
When a fault is discovered in a communications systems such as SDH (synchronous digital hierarchy) or SONET (synchronous optical network) an alarm is generated from the network element detecting the fault, identifying the network element as the source of the alarm, so that the system can be reconfigured to avoid routing signals through the affected link and/or network element, and so that action can be taken to restore normal communication. For example, a network element may detect a loss of signal on a link and generate a LOS (loss of signal) alarm. The alarm is passed up to the network element manager.
Since such systems include virtual connections which pass through several physical links (known as a xe2x80x9clayer hierarchyxe2x80x9d), it is also necessary to generate secondary alarms (alarm indication signal (AIS) alarms) from the network elements downstream of the fault. Thus, the network element that detects the loss of signal sends a characteristic signal (an AIS signal), which consists of a recognisable pattern, such as a series of xe2x80x981xe2x80x99 bits, along its downstream link to inform downstream network elements that a loss of signal has been detected. The network elements that receive such an AIS signal generate AIS alarms which are sent up to the element manager. The network element manager sends the alarms on up to the sub-network manager. The generation of these secondary alarm signals means that a single fault can result in a great many secondary alarm signals.
An analogy is with a railway network, in which many routes pass through any one section of track, and each route also passes through many sections of track. Therefore, if one section of track is damaged, this affects many stations along the various routes. One damaged section of track will therefore give rise to announcements in many stations.
Whilst these secondary (AIS) alarms perform an essential function, they also give rise to a problem, in that, from the point of view of the sub-network manager, there are too many alarms raised as a result of a single fault. The sub-network manager needs to report and keep account of faults and to maintain a map of the system configuration, but it is not involved in routing particular messages through the system, so the fewer signals that correspond to a single fault the better; one signal per fault would be ideal. However, the element managers only deal with the network elements as individual elements, and do not have the high-level information to distinguish between signals that the sub-network manager needs and those which it does not need, so they have to send them all to the sub-network manager. It is not enough simply to send primary, root cause, alarms, such as LOS alarms, and to withhold secondary (AIS) alarms, since the source of the primary signal may not be in the sub-network manager""s domain, although the fault affects the operation of the sub-network manager""s domain (i.e. the portion of the network that it controls).
Therefore a method is required which distinguishes between the secondary (AIS) alarms that the sub-network manager needs and those that it does not need.
According to the principles of the invention there is provided a method of filtering secondary alarm signals sent to a sub-network manager of a telecommunications network wherein the sub-network manager is arranged to manage network elements in a respective managed domain. More specifically, the method comprises determining whether the alarm is a secondary (AIS) alarm generated in response to a signal received by a network element on an incoming link. If the alarm is not a secondary alarm, the alarm is passed for further processing. If the alarm is a secondary alarm, a search for the upstream termination point of the incoming link is carried out to determine whether the upstream termination point is in the managed domain of the sub-network manager. If the upstream termination point is in the managed domain, the alarm is blocked. If not in the managed domain, the alarm is passed for further processing.
Accordingly, since the sub-network manager has information on the connectivity of the network domain, it can determine whether the network element that originates the alarm is on the edge of the managed domain. If the upstream link is terminated by a termination point which is not in the managed domain, the alarm is passed. If it is terminated by a termination point which is in the managed domain the alarm is blocked. Thus, the only secondary alarms that are passed are those that are generated as a response to a signal entering the domain from outside.